In the past, golf balls were manufactured by molding a molding material in a mold cavity having a suitable surface pattern. For example, one - piece golf balls were often manufactured by supplying a molding material into a mold cavity. Multi - layer golf balls (e.g., two - piece golf balls) and thread - wound golf balls were often produced by placing a preformed core in a mold cavity at its center and supplying a molding cover material into the space between the mold cavity surface and the core, followed by, molding. In either case, there were obtained golf balls having a surface pattern transferred from the cavity surface pattern.
These methods for molding golf balls used molds which were most often fabricated by precision casting and pressing.
The precision casting method is described with reference to the accompanying figures, FIGS. 14 to 17. The method includes the steps of first making a male master model a of brass or the like as shown in FIG. 14, forming a silicone rubber reverse mold b as shown in FIG. 15 therefrom, forming a gypsum mold c as shown in FIG. 16 therefrom, and finally forming a golf ball manufacturing mold d of beryllium copper or tool steel as shown in FIG. 17 from the gypsum mold c. The pressing method is designed to fabricate molds by pressing a master model of hard alloy having dimple - shaped depressions into a semi - spherical cavity of a preformed mold free of dimple - forming projections under high pressure, thereby transferring the dimple shapes to the latter in the plastic deformation region. The molds are usually made of zinc alloy or stainless steel.
Nevertheless, the mold fabricated by the prior art precision casting and pressing methods are unsatisfactory in precision. More particularly, the precision considerations for golf ball manufacturing molds are mold sphericity and the precision of all dimples in a single mold (precision of transfer from the master). In the precision casting method, sphericity is adversely affected by cooling shrinkage upon casting. In addition, for great many reversal operations from the master model until the fabrication of final molds and other reasons, the dimples in the final molds obtained from the same master model significantly vary in size and such size variations are inconsistent among dimples in a single mold and among molds. On the other hand, the pressing method fabricates molds by utilizing the plastic deformation of metal as mentioned above. Since spring - back always takes place on the material surface due to stress release upon removal of the master model, it is basically difficult to reproduce dimple shapes faithful to the master model dimple shapes, resulting in insufficient dimple precision. From a sphericity standpoint, removal of the master model is always accompanied by undercutting which results in insufficient sphericity at the mold parting line surface.
Attempts were made to utilize electro-forming for fabricating of golf ball-manufacturing molds. Molds are fabricated by first forming a master model of brass or the like, conducting electroforming on the master model, and thereafter dissolving away the master model, leaving the electroformed layer serving as the mold. This electroformed mold is fully improved in precision. However, when it is desired to use a mold having a plurality of cavities for manufacturing a corresponding plurality of golf balls, this electroforming method requires a plurality of master models of the same shape. In addition, after an electroformed mold has been fabricated, the mold has to be machined with necessary accessories including post - machining aids such as a post-machining reference surface and molding aids such as a parting surface, pin aperture(s), runner, gate, and spew, greatly adding to the cost.